1. Field of the Invention
The present invention relates to a field of optics, and more particularly to an optical compensation film group and method for reducing light leakage of a vertical alignment LCD using the same.
2. Description of Related Art
As the increasing viewing angle of the Thin-Film-Transistor LCD (TFT-LCD), the contrast of the screen is reduced. The sharpness of the screen is also gradually decreased. This is due to the bi-refractive index of the liquid crystal molecules in the liquid crystal layer changing with the observation angles. By using a wide viewing angle compensation film for compensating, it can effectively reduce light leakage of the screen in the dark state, and within a certain angle, it can greatly improve the contrast of the screen.
The compensation principle of the compensation film is generally correcting the retardation caused by the liquid crystal molecules in the different viewing angles so that the birefringence property of the liquid crystal molecules can obtain a symmetric compensation.
The compensation films are different for different liquid crystal display modes. The compensation film used in large-size liquid crystal television is for VA (vertical alignment) display mode LCD. In the early, it uses Konica Company's N-TAC film. Subsequently, it uses OPOTES Company's Zeonor film, Fujitsu's F-TAC series films, or Nitto Denko's X-plate film. Conventionally, the common compensation structure with a single biaxial compensation film is shown in FIG. 1 and with a double biaxial compensation film is shown structure shown in FIG. 2. As illustrated, the structure with the single biaxial compensation film is usually followed by a TAC (cellulose triacetate) compensation film, a PVA (polyvinyl alcohol) compensation film, a TAC (cellulose triacetate) compensation film, a PSA (pressure sensitive adhesive) compensation film, a VA (vertical alignment) cell unit, a PSA (pressure sensitive adhesive) compensation film, a biaxial compensation film, a PVA (polyvinyl alcohol) compensation film, a TAC (cellulose triacetate) compensation film. In the compensation structure, there is only one biaxial compensation film so that it is called the compensation structure with a single biaxial compensation film.
By using the double biaxial compensation film for compensation, it can effectively reduce the light leakage of the screen in the dark state so as to improve the contrast and sharpness at a large viewing angle. However, its price is more expensive, and is not conducive to reduce costs. By using the compensation structure with a single biaxial compensation film, it can effectively reduce the costs, but may increase the light leakage of the screen in the dark state.
For example, FIG. 3 is a light leakage distribution diagram in dark state generated by a compensation structure with a single biaxial compensation film in the prior art. Wherein, an optical path difference (LCΔnd)=324.3 nm, a liquid crystal pretilt angle=89°, a compensation value Ro of an in-plane optical path difference of the biaxial compensation film=72 nm, a compensation value Rth of an out-of-plane optical path difference of the biaxial compensation film=240 nm, a compensation value Rth of the TAC compensation film=23.6 nm.
As shown FIG. 3, the light leakage of the compensation structure with a single biaxial compensation film is very serious.
How to compensate using the compensation structure with a single biaxial compensation film and to achieve a good compensation effect of the light leakage so as to reduce the costs and the light leakage is a technical problem that needs to be solved.